Many wireless systems today apply burst transmission; short, repetitive bursts are used to carry the information from transmitter to receiver. An advantage of burst transmission is that the transmit and receive circuitry only has to operate during the presence of the burst. In between the bursts the transceiver can be put in standby mode, thus saving power. This is in contrast to continuous wave (CW) modulation like FM or spread-spectrum transmission where the transceiver has to operate continuously. Burst transmission is therefore attractive in battery driven, portable devices where power consumption is of crucial importance.
Current cellular systems like GSM (Global System for Mobile Communication) and D-AMPS (Digital Advanced Mobile Phone System) and office communication systems like DECT (Digital European Cordless Telecommunications), are built to provide multiple user access on a common air interface by applying TDMA (Time Division Multiple Access). The time is divided into time slots which represent the channels. Each user has its own time slot or set of time slots. However, to avoid interference, time slots should be strictly separated and no overlap may occur. This requires accurate synchronization of the transceivers that are part of the TDMA network.
A further example of a system which uses some kind of time slots in the transmission protocol is FH-CDMA (frequency-hopping code-division multiple access), in which the frequency spectrum is divided into a large number of frequency bands, so called hop channels. Each transceiver is instructed to jump from hop channel to hop channel according to a unique hopping sequence. Different links should use different hopping sequences which are mutually orthogonal, so that if a user occupies one hop channel, no other user occupies this hop channel at the same time. In order to keep the hopping sequences orthogonal, time synchronization of the transceivers is required.
In all existing wireless slotted radio communication systems the method for synchronization is based on an internal clock timer in a central station, to which all other stations in the system adjust their internal timers. In mobile telephony systems like GSM, the base stations transmit fixed synchronization signals, also called beacon (broadcast control) signals, to which all mobile transceivers can synchronize. By appropriate channel allocation, each user gets its own time slot and the channels remain orthogonal.
Wireless systems in which no central synchronization or control exists are typically applied in private, short-range communications, and preferably make use of unlicensed frequency bands such as the ISM (Industrial, Scientific, Medical) band. In the literature such clusters of independent transceivers which share the same air interface without the supervision of a master control unit are known as ad-hoc networks. Transceivers can communicate with each other on a point-to-point or point-to-multipoint basis but have very little knowledge or no knowledge whatsoever of the communications between other transceiver arrangements. However, all transceivers do make use of the same common air interface. As a result, there is a contention problem; without synchronization, links will mutually interfere if their burst transmission overlaps in time and collisions occur. Typical examples of wireless connections in an ad-hoc network are wireless links between computers and local area networks (LAN), and wireless connections between fixed, mobile or portable phones and their peripherals. Examples of the latter are laptop-phone or phone-headset connections.
To deal with the collision problems, communication protocols have been developed that apply retransmission. If a burst is not received correctly, the transmitter retransmits the same information. This procedure is repeated until the receiver acknowledges the correct reception. A frequently used protocol working according to this procedure is the ALOHA protocol. Retransmission protocols are popular in packet radio systems, where the information is arranged in data packages each having an address and an order number. If, due to a collision, a data package is lost, it is retransmitted later on and can be inserted into the package series due to its unique order number.
The ALOHA protocol works well as long as the number of users is much smaller than the number of channels. Then the probability of a collision is small and both the throughput and delay per link remain acceptable. In combination with the ALOHA protocol, it is nowadays common practice to use collision avoidance methods. In this case, the transceiver first listens to a particular channel before it transmits. If the transceiver measures activity, it waits an arbitrary period of time and then listens again until no activity is observed. Then the transceiver starts the transmission. This technique, often referred to as CSMA (Carrier Sense Multiple Access), reduces the number of collisions, that can still occur due to delay differences.
Another variant of the ALOHA protocol is called slotted ALOHA. In slotted ALOHA systems, the time scale is divided into intervals of equal lengths. Every user wishing to transmit has to synchronize its transmission so that it starts at the beginning of an interval. It can be shown that the throughput of a slotted ALOHA system, under certain conditions on the traffic intensity, is twice as high as the maximum throughput of a pure ALOHA system. The importance of a proper synchronization of the individual transceivers for this slotted ALOHA system is apparent.
For applications in which the links can transfer data and/or speech, packet radio techniques and ALOHA protocols are not attractive. Speech communication does not have the same properties as data communication has, i.e., it is continuous, delay sensitive, and speech samples must be received in the right order (within a reasonable delay window). Retransmissions are impossible since it would give an accumulated delay at the receiver, which means that the error rate on the speech link must remain acceptably low. Therefore, the collision probability must be low, which is only accomplished when the number of users is much smaller than the number of channels. Therefore, the number of users that can use the common air interface is rather limited. Even then, problems may arise if the burst repetition rates of two users drift towards each other, giving continuous collisions during a certain period until the bursts have drifted from each other again. Without synchronization, therefore, a certain degree of interference due to mutual drift is inescapable.
For the applications mentioned above, where we have links carrying speech, data, or both, without a common master control, there is a strong request to keep the channels orthogonal. This requires synchronization between the transceivers that share the common air interface. In addition, consistent channel allocation should be applied to avoid two users accessing the same channel.
The patent publication U.S. Pat. No. 5,285,443 describes a method for synchronization of multiple base stations for cordless telephony within a defined geographical area. One of the base stations is configured as a master station and the remaining base stations are configured as slave stations. The master station transmits a synchronization signal to which the slave stations synchronize. In case the synchronization signal from the master station is missing, one of the slave stations will change from slave mode to master mode and start to transmit a synchronization signal to which the other base stations can synchronize. The described synchronization procedure thus synchronizes the base stations; the cordless telephones, with which the base stations communicate by applying TDD (Time Division Duplex), must separately be synchronized by the base stations. Problems with interference can easily occur with this synchronization technique at the borders of the synchronization area. Transmissions from transceivers located in the periphery of the area in which the synchronization signal can be received, can be received from transceivers outside this synchronization area and thereby cause interference. These peripheral transceivers are for the same reasons also vulnerable for interference. Hence, this synchronization technique requires careful planning not to cause interference problems.
The patent publication U.S. Pat. No. 5,124,698 describes a method for synchronization of base stations in a paging network. A first master base station transmits a synchronization message to at least one neighbour base station. This base station reads the synchronization message, uses the received information for synchronization, and transmits a new synchronization message that other base stations can use for synchronization. This procedure is repeated following a predetermined route until all base station are synchronized. Hereby, the base stations use information from only one other base station each for synchronization. The described synchronization method of the base stations requires a detailed route planning. The method is thus only applicable for static systems without central control.